A measuring circuit such as that shown in FIG. 2 has heretofore been employed to measure the impedance or the like of a DUT using a desired signal with a DC bias voltage being applied to the DUT. In the figure, a capacitor 212 and an inductor 211 are provided to prevent a null detecting amplifier 203 from being saturated by a large DC signal which would otherwise flow through a current detecting resistor Rr from a DC bias voltage source 208. The capacitor 212 is employed to block DC signals, while the inductor 211 is employed to constitute a return path of DC signals. A measurement signal from a measurement signal source 206 is added to a DC signal from the DC bias voltage source 208 by an adding means 207 and added to one end of a DUT 205. On the other hand, the other end (the point G) of the DUT 205 is virtually grounded in an alternating current manner by the null detecting amplifier 203; therefore, the signal voltage from the measurement signal source 206 directly represents the voltage across the DUT 205. Ideally, all the measurement signal flowing through the DUT 205 flows through the current detecting resistor Rr. Thus, if the voltage across the DUT 205 is represented by VAD and the voltage across the current detecting resistor Rr by VAR, the impedance ZDUT of the DUT 205 may be expressed as follows: EQU ZDUT=Rr.VAD/VAR (VAD and VAR are vector voltages)
However, the magnitude of inductance of the inductor 211 has its limit. Particularly in a low-frequency region, no sufficiently large impedance can be obtained, so that part of the measurement signal flows outvia the inductor 211, resulting in an error in the measurement of impedance. Further, the DC resistance of the inductor 211 causes a DC offset voltage to be generated at the point G by the DC bias current flowing through the inductor 211, which results in an error in the DC bias voltage applied to the DUT 205.
To solve these problems, Japanese Utility Model Application No. 63-29322 (1988) discloses an arrangement wherein an active simulated inductor such as that shown in FIG. 3 is employed in place of the passive inductor 211. Since the simulated inductor can have a considerably large inductance and also has a small DC resistance, it is possible to conduct measurement with a high degree of accuracy even when the impedance of a DUT is measured with a high DC bias voltage being applied to the DUT. It is also possible to set a DC bias voltage with a high degree of accuracy.
However, when the DUT 205 is connected to a circuit element measuring apparatus 10 through four coaxial cables H.sub.CUR, H.sub.POT, L.sub.POT (length: 1 m, for example) to measure a circuit parameters, for example, impedance, of the DUT, as shown in FIG. 4, some problems arise. First of all, when a simulated inductor is connected to the point L in the figure, the simulated inductor must be disposed within a measuring jib (test fixture) employed to detachably secure the DUT 205, which leads to a rise in the cost of the test fixture and an increase in the size thereof. When a simulated inductor is connected to the point S in the figure, the AC potential at the point S is affected by the inductive and resistive components of the cable L.sub.CUR so that it becomes different from the potential (approximately equal to the ground potential) at the point L. Accordingly, the measurement current flows through the simulated inductor, resulting in an error in the measurement current flowing through the current detecting resistor Rr, which leads to an error in measurement. This is because the impedance of the simulated inductor cannot be increased due to the reasons that the electrical characteristics of the simulated inductor are equal to those of the voltage controlled current source (VCCS) 303 at high frequencies above the band width of the loop and that a high power transistor having a large stray capacitance is employed for the VCCS in order to increase the amount of absorption of DC current. In addition, the larger the amount of current absorbed by the simulated inductor, the larger the potential difference between the DC potential at the Point L and the DC potential (approximately equal to the ground potential) at the point S due to the effect of the resistive component of inner wire of the cable L.sub.CUR. Therefore, an error is invited in setting of a DC bias voltage applied to the DUT 205.